During the production of drinking water, “alum” (aluminium sulphate) is typically used to clarify the water. The alum acts by catching fine suspended particles in a gel-like precipitate of aluminium hydroxide. This results in the production of a by-product known as “alum sludge”, otherwise known as water treatment sludge or aluminium drinking water treatment residues (Al-WTR), which may be defined as the “accumulated solids or precipitate removed from a sedimentation basin, settling tank, or clarifier in a water treatment plant”. The alum sludge must be managed and disposed of by the water treatment plant operator. Presently, most alum sludge is disposed of to landfill, although where the sludge comprises a solids concentration of 25% or less, disposal by way of land application (e.g. to croplands, marginal lands for reclamation, forestry lands or other dedicated sites) is more usual (1). However, these processes can be expensive and, additionally, results in the loss of a potentially valuable material. The recovery of alum from alum sludge has been proposed as one way to reduce the costs (2), however to date, this has not been widely employed. The present applicants herein propose an alternative and novel strategy for “disposing” of alum sludge in a manner which allows the reuse of the aluminium content. That is, the present applicants have surprisingly found that alum sludge may be used, in an unrefined form (i.e. without any particular recovery of the alum), for the stabilisation of contaminants in soils (e.g. soil from former industrial sites being redeveloped for residential or other use) and other solid or semi-solid materials. Some alum sludges also contain a reactive carbon material (e.g. activated carbon which is sometimes used in water treatment processes) and the reactive carbon material may also assist in the stabilisation of contaminants.